The present invention relates to a slip-controlled brake system provided for automotive vehicles comprising a braking pressure generator which is substantially composed of a power brake booster supplied with auxiliary energy and containing a control valve and a booster chamber, and a master cylinder directly connected to the booster and acted upon by the booster pressure. Several separate pressure fluid circuits connect the rear-wheel brakes to the power brake booster and the front-wheel brakes to the master cylinder with the circuits containing inlet valves which normally assume an opened position and are adapted to be closed for slip control. Normally closed lines are provided for the return flow of pressure fluid out of the wheel brakes to a pressure supply reservoir, and a normally closed pressure fluid conduit connects the power brake booster to the master cylinder through which pressure fluid is supplied out of the booster chamber by way of pre-chambers into the working chamber of the master cylinder upon response of the brake slip control.
A brake system of this type is described in German printed and published patent application 30 40 561. In this case, the braking pressure generator consists of a hydraulic power brake booster, the braking pressure control valve of which is mechanically coupled to the brake pedal. The pressure which is pedal-responsively introduced and built up in a pressure chamber within the braking pressure generator acts, on the one hand, on a single-type or tandem-type master cylinder which is part of the braking pressure generator and, on the other hand, directly on the wheel brakes at the rear axle of the vehicle. The working chamber of the master cylinder or, respectively, the two working chambers if a tandem master cylinder is concerned, is/are in hydraulic communication with the front-wheel brakes. To control brake slip by way of keeping the braking pressure constant or by decreasing it upon the occurrence of an imminent locked condition, there are inserted inlet valves into the pressure fluid lines to the brakes and outlet valves into hydraulic lines which connect the wheel brakes with the pressure supply reservoir. As soon as there is no need for brake slip control, the inlet valves adopt the opened position, while the outlet valves are in the closed position.
When designing the various components of the brake system, such as the braking pressure generator, the wheel brakes, the auxiliary energy supply system and the distribution of the pressure onto the rear axle and the front axle, those conditions prevailing in the "normal case", i.e. without the action of the brake slip control system, must be considered as well as the requirements arising in the event of panic stops or other situations in which brake slip control is put into operation. In addition, the brake system is required to be dimensioned such as to ensure that there will be maintained a force distribution as favorable as possible in the event of over-braking and de-activation of brake slip control, for example, a disturbance in the slip control system.
In this respect, specific legal provisions have to be complied with. For instance, in the event of overbraking it should be provided that for reducing the danger of skidding the rear wheels are allowed to lock, but only after the front wheels.
As the conditions and requirements imposed on the brake system are differing in the case of "normal" braking operation and during brake slip control, difficulties are involved when designing the brake system in finding an acceptable compromise.
It is therefore an object of the present invention to overcome the shortcomings of these prior art systems and to better adapt a brake system of the type described to the various situations prior to and after the commencement of slip control by simple means and by necessitating only little additional effort.